Poly(ethylene-2,6-naphthalene dicarboxylate), referred to as PEN, is widely used as an extrusion and injection-molding resin because of its good heat resistance, high glass transition temperature, and gas barrier properties. PEN is used in the fabrication of various articles for household or industrial use, including appliance parts, containers, and auto parts. One major drawback of PEN, however, is its inherent bluish fluorescence. Thus, objects prepared with PEN have a hazy and bluish appearance. This phenomenon is especially of concern in the packaging of foods and beverages wherein the food or beverage inside the PEN container appears unnatural.
Fluorescence is a type of luminescence in which an atom or molecule emits radiation in passing from a higher to a lower electronic state. The term is restricted to phenomena in which the time interval between absorption and emission of energy is extremely short (10.sup.-10 to 10.sup.-6 second). Fluorescence in a polymer or small molecule, occurs when a photon is emitted from an excited singlet state. Quenching of fluorescence eliminates or reduces the ability for photon emission by providing an alternative pathway for the excited state energy such as vibronic or heat loss, or intersystem crossing to the excited triplet state.
Methods to quench fluorescence in PEN have been disclosed by Chen Shangxian et al. in an article entitled, "Fluorescence Spectra of Poly(Ethylene-2,6-Naphthalene Dicarboxylate)" which appeared in SCIENTIA SINICA, Vol. XXIV, No. 5, May 1981, and by CAO Ti et al. in an article entitled, "Intermolecular Excimer Interaction In Poly(Polytetramethylene Ether Glycol Aryl Dicarboxylate)" which appeared in ACTA CHIMICA SINICA, Vol. 42, No. 1, 1984. Both of the references disclose the use of o-chlorophenol to quench PEN fluorescence in a chloroform solution. Dissolving the PEN in a chloroform solution to disperse the fluorescence quencher therein, however, is not practical on an industrial scale because only very dilute PEN solutions can be prepared. In addition, the PEN must have a low molecular weight to dissolve in the chloroform solution.
In contrast, the present inventors have unexpectedly determined that the incorporation of 0.1 to 5 mole percent of a copolymerizable halogen containing aromatic compound in PEN significantly reduces fluorescence without deleteriously effecting the physical properties of the polyester.